Halliday & Matthiessen (2014: 267):
In the ‘attributive’ mode, an entity has some class ascribed or attributed to it. Structurally, we label this class the Attribute, and the entity to which it is ascribed is the Carrier — the ‘carrier’ of the ‘attribute’. … This type of clause is a resource for characterising entities serving as Carrier; and it is also a central grammatical strategy for assessing by assigning an evaluative Attribute to a Carrier.